Conventionally, a film projector for irradiating a 35 mm (millimeters) film with light through an aperture so as to project an image on a screen is generally used in a movie screening system of a movie theater. FIG. 9 is an explanatory diagram of the structure of a projector for a movie film. Image frames (hereinafter referred to as merely frames) containing continuous content are recorded on a film 71 at predetermined intervals. This film 71 is transported by a transport mechanism (not shown in the figure) and passes through a picture gate unit 72 from an upper part to a lower part thereof. Light emitted from a light source apparatus 73 is condensed and passes through an aperture formed in the picture gate unit 72, so that a frame recorded on the film 71 is irradiated with light. The size of each frame of the film 71 is, for example, approximately 24×18 mm and a diagonal thereof is approximately 30 mm. Therefore, in order to efficiently irradiate the area of the film, the light source apparatus 73 is required to have a structure in which the light from a light source lamp is efficiently condensed so that the light may incident within a circle whose diameter is approximately 30 mm at the picture gate unit 72.
Therefore, the light source apparatus 73 includes a xenon short arc lamp 73a (hereinafter referred to as a xenon lamp), which serves as a light source lamp, and a reflection mirror 73b which is arranged at a rear portion thereof. And, the reflection mirror 73b has a reflective surface made up of a spheroidal surface, which condenses the light emitted from the xenon lamp 73a so that the light may incident within a circle, wherein, as mentioned above, the diameter of the circle is approximately 30 mm. As shown as an optical path in the figure, the light emitted from the xenon lamp 73a is reflected by the reflection mirror 73b, is condensed at a second focal point (F2), passes though the film 71, is expanded by a projection lens 74, and is projected on a screen 75.
However, the optical path shown in this figure is ideal in such a system. However, an arc of the xenon lamp 73a does not actually serve as a point light source, but has a finite size in fact. For this reason, the light from the arc is not condensed at one point, so that an inside area of a circle having a certain size is irradiated with light at a position of the second focal point. And it is known that, in case the same ellipse mirror is used, an irradiated area at the position of the second focal point becomes large approximately in proportion to a cross section area of an arc (an area of the arc when viewed from a side thereof).
Given such a situation, a xenon lamp, in which an arc length is approximately 3-7 mm, is used as a light source for a film projector, in order that the inside area of the circle having a diameter of approximately 30 mm is irradiated with light. In addition, the “arc length” is equal to the distance between electrodes at a time of steady lighting of a lamp. Furthermore, a numerical example of the specification of such a xenon lamp for a film projector will be given below. For example, the rated power consumption thereof is 0.9-6.0 kW, a diameter at a tip of a cathode is 0.6-1 mm, the pressure of enclosed xenon is 0.6-0.9 MPa, the current density at the tip of the cathode is 76-110 A/mm2, and the bulb wall loading thereof is 18-29 W/cm2. In the above example of the specification, when concrete numerical values of the xenon lamp for a film projector whose rated power consumption is 4 kW, numerical values will be given below. The arc length is 6 mm, the diameter of the tip of the cathode is 0.9 mm and the pressure of enclosed xenon is 0.7 MPa, the current density thereof is 108 A/mm2, and the bulb wall loading thereof is 25 W/cm2. In addition, in the above description, the “current density” means current density which is obtained by dividing lamp current by a cross section area of the cathode at a position of 0.5 mm from the tip of the cathode, and the “bulb wall loading” means electric power per unit area, which is obtained by dividing lamp electric power by the inner surface area of an arc tube portion.
Since the xenon lamp emits high intensity light, the temperature of the tip of the electrode becomes extremely high. For this reason, the tip of the cathode that emits electrons is consumed intensely. When the tip of the electrode is worn out so that unevenness is formed on a face of the tip of the cathode, a phenomenon commonly referred to as “flicker” occurs in which a starting point of arc electric discharge moves between a convex portion and another convex portion. When this flicker occurs, the luminance distribution of the lamp fluctuates so that it appears as flickering on a screen.
In order to prevent occurrence of such a flicker (i.e., in order to obtain the stable radiation light over an extended time period), improvement in such a xenon lamp has been repeated. For example, tungsten, to which thoria (ThO2) whose melting point is high even in electron emissive material is added, is used for a cathode, and a carbonization layer with a thickness of 8-30 μm (micrometers), which is made of tungsten carbide (W2C), is formed thereon except the vicinity of the tip thereof. By forming this carbonization layer thereon, the electron emissive material (for example, thoria (ThO2)) added in the cathode is reduced by carbon, thereby generating thorium (Th) at time of lamp lighting, so that the thorium (Th) can be efficiently supplied to the tip face of the cathode. Such technology is disclosed, for example, in Japanese Patent Application Publication No H10-283921.
The above-mentioned carbonization layer is not (should not be) formed on the tip portion of the cathode. This is because an area of the tip portion of the cathode reaches high temperature, for example, approximately 2,900° C., so that if tungsten carbide (W2C), whose melting point is low, exists therein, it melts at an early stage, whereby the electrode is worn out, or the arc tube is blackened, so that the intensity of radiation light decreases, and thus the lamp come to the end of its life span at an early stage. In a xenon lamp, which is optimized by applying such technology to the above described lamp for a film projector, the quantity of carbon is in a range of 0.5-1.8 μmol/cm3 per unit internal volume of the arc tube.
Moreover, silica glass is usually used for such an arc tube. Therefore, since problems, such as a rise of starting voltage or blackening of the arc tube, may arise, when water, which is originated from OH groups contained in the silica glass, is discharged in the lamp with lighting of the lamp, the arc tube in which the OH group concentration is low is generally used. The OH group concentration of such an arc tube is maintained to a level of raw material in a state of a pre-formation thereof, by using dried gas (N2) in a forming step of blowing up the arc tube.
As the result of these improvements, a life span of the xenon lamp as to a flicker reaches approximately 3,500 hours. Thus, it is possible to sufficiently realize a long usage life thereof, since the starting nature of the lamp has been improved and the problem of the blackening has been improved.
In addition, in recent years, in the movie screening system of a movie theater, the advanced computer graphics using the digital technology, by which the quality of an image is improved, can be realized. Therefore, since there are advantages that there is no degradation of the film, and costs accompanying film production can be reduced, the digital cinema becomes widespread. In accordance with the spread, the digital projector which uses a DLP (Digital Light Processing: Registered Trademark) technology is replacing the old system at a rapid pace.
An example of the structure of such a digital projector is shown in FIG. 10. In this digital projector 80, light from a xenon lamp 81 is condensed by a reflection mirror 82 having an ellipse reflective face, and irradiates image elements called a DMD (Digital Micromirror Device: Registered Trademark) through a color filter 83, an integrator rod 84, and condensing lenses 85a and 85b. The light reflected by the DMD 86 is projected on a screen 88 by a projection lens 87, so that an image is shown thereon.
In such a digital projector 80, the light from the xenon lamp 81 must be condensed at high efficiency so as to be incident on an end face of the integrator rod 84. Thus, the light must be condensed at high efficiency, because the end face of the integrator rod 84 usually has a size which is comparable with the DMD 86 in which a diagonal line is as short as a 0.7-1 inch (17.8-25.4 mm), so that in order to project an image with brightness comparable with that of a conventional projector for a movie film, on a screen, the light must be condensed in a small area within a range of 35-70% of an area in the case of the projector for a movie film.
Since an area irradiated by the reflection mirror 82 is approximately proportional to a cross section area of an arc, it is necessary to use the xenon lamp in which the arc length is shorter and the pressure of enclosed xenon is further increased in order to make the arc thin in the xenon lamp 81 for a direct projector. Consequently, the arc length of the xenon lamp 81 is set to approximately 2-7 mm, and 1 MPa or more of the pressure of the enclosed xenon gas is required at a normal temperature wherein specifically, the pressure thereof in the range of 1-2 MPa thereof is required. And in order to bear the high pressure in an operation at a time of lamp lighting, it is necessary to miniaturize the arc tube so as to be smaller than that of the prior art, and thereby the bulb wall loading of the xenon lamp for a digital projector increases to 30 W/cm2 or more, and specifically the bulb wall loading within a range of 30-40 W/cm2 is required. This is remarkably high, even compared with a conventional xenon lamp for a film projector.
In addition, shortening of a distance between the focal points of an ellipse reflective face (a distance between F1 and F2) may be also considered as a means for making small the area irradiated with light from the reflection mirror 82. However, this method cannot be adopted in the above described case, since the rate of rays which have a large angle with respect to an optical axis 89 increases, so that the light which does not reach the DMD element increases, whereby the utilization ratio of light decreases. In other words, when the irradiated area becomes small, it is difficult to raise the condensing efficiency thereof by only devising an optical system.
Furthermore, it is necessary to increase an optical output of the xenon lamp 81 because of a demand on a brighter image of a digital projector. For this reason, from a viewpoint of reducing rays from the arc which are blocked by the cathode, a diameter of the tip of the cathode is required to be smaller than that of the prior art. Therefore, the diameter thereof is, for example, 0.35-0.7 mm, so that the diameter of the tip of the cathode of the lamp for a digital projector is smaller than that of the prior art. Consequently, the current density of the tip of the cathode also becomes high, specifically 119 A/mm2 or more, and particularly it is in a range of 119-210 A/mm2.
In an example of such specification, more concrete numerical values of the xenon lamp for a digital projector, in which the rated power consumption is 4 kW, will be given below. The arc length thereof is 3.5 mm, the diameter of the tip of the cathode is, 0.6 mm, the pressure of enclosed xenon is 1.8 MPa, the current density thereof is 119 A/mm2, and the bulb wall loading thereof is 37.5 W/cm2. In addition, as mentioned above, the “current density” means current density that is obtained by dividing lamp current by a cross section area at a position of 0.5 mm from the tip of a cathode, and the “bulb wall loading” means electric power per unit area, which is obtained by dividing lamp electric power by an inner surface area of an arc tube portion.
The features of such a xenon short arc lamp for a digital projector are summarized below. The pressure of enclosed xenon gas is high; the bulb wall loading thereof (a value which is obtained by dividing lamp electric power by an inner surface area of a portion where the arc tube is swollen) is high as a result of miniaturizing an arc tube in order to bear the high operation pressure; and the current density thereof becomes high as a result of making small the diameter of the tip of the cathode. Concrete numerical values about the above case will be given below. The pressure of enclosed xenon gas is 1 MPa or more, the bulb wall loading thereof is 30 W/cm2 or more, the current density of the tip face of the cathode is 119 A/mm2 or more. Thus, the very severe specification is required. And when the above requirements of specification is satisfied, the temperature of the tip of the cathode of the xenon lamp rises further, so that consumption and deformation of the tip portion of the cathode makes remarkably rapid progress, and after lamp lighting, the tip face of the cathode becomes large and unevenness is formed thereon, whereby a flicker occurs at an early stage in a short time. And, in the conventional technology, for example, even if the life span as to flicker of the xenon lamp is improved according to formation of a carbonization layer or adjustment of the shape at the tip of the cathode, a life span thereof comes to the end in very short period of only 200-350 hours after it is lighted.